(A) Field of the Invention
The present invention relates to a flash memory structure and method for fabricating the same, and more particularly, to flash memory structure having a V-groove profile and the method for fabricating the same.
(B) Description of the Related Art
Flash memory has been widely applied to the data storage of digital products such as laptop computers, digital assistants, cell phones, digital cameras, digital recorders, and MP3 players. Recently, a flash memory comprises a silicon-oxide-nitride-oxide-silicon (SONOS) structure, which is widely used in flash memory since it possesses the advantages of a thinner memory cell and a simpler fabrication process.
FIG. 1 illustrates a flash memory cell 10 with a SONOS structure according to the prior art. The flash memory cell 10 comprises a silicon substrate 12, two doped regions 14 and 16, a tunnel oxide layer 22, a silicon nitride layer 24, a silicon oxide layer 26, and a polysilicon layer 28. Particularly, the SONOS structure consists of the silicon substrate 12, tunnel oxide layer 22, the silicon nitride layer 24, the silicon oxide layer 26, and the polysilicon layer 28. While charge-trapping site in the silicon nitride layer 24 can capture electrons or holes penetrating the tunnel oxide 22, the silicon oxide layer 26 serves to prevent electrons and holes from escaping the silicon nitride layer 24 to enter into the polysilicon layer 28 during writing or erasing operations of the flash memory.
When the polysilicon layer 28, serving as the gate electrode, is charged to a positive potential, electrons in the silicon substrate 12 will inject into the silicon nitride layer 24. Inversely, a portion of electrons in the silicon nitride layer 24 will be repulsed to inject into the silicon substrate 12 to form holes in the silicon nitride layer 24 when the polysilicon layer 28 is charged to a negative potential. Electrons and holes trapped in the silicon nitride layer 24 change the threshold voltage (Vth) of the flash memory cell 10, and different threshold voltages represent that the flash memory stores different data bits, i.e., “1” and “0”.